This invention relates generally to time-domain reflectometry and more particularly to a time-domain reflectometry apparatus and method for testing a terminated network cable.
In a network environment, such as Ethernet systems, devices are connected to the network by cables, commonly referred to as link cables. FIG. 1 illustrates a portion of a network where device 18, such as a computer or printer, is connected at node 16 to one end of link cable 14. The other end of link cable 14 is connected at node 12 to the network, which in FIG. 1 is represented by cable 10.
Technicians troubleshooting networks need to test, among other network elements, the various link cables in the system. To verify that link cables meet specifications, technicians use cable test instruments, which have the ability to test, among other parameters, cable length. In an Ethernet system, for example, the length of a link cable is limited by system standards to a maximum of 100 meters. Cable test instruments typically employ time domain reflectometry (TDR) to measure the length of link cables. In FIG. 1, cable tester 20 is connected to link cable 14 at node 16. From this location, cable tester 20 may perform TDR testing on link cable 14.
FIG. 2 illustrates a typical TDR circuit 22 such as may be utilized in tester 20 and found in the prior art. TDR circuit 22 is coupled to terminals 24, 26 of instrument 20. A link cable 28, which is to be tested, is connected to terminals 24, 26. Cable 28 is illustrated as a twisted pair, that is, a pair of wires 28a, 28b terminated at one end to terminals 24, 26. Link cable 28 has a characteristic impedance, Z0.
TDR circuit 22 includes voltage sources V1 and V2 coupled between ground and resistors R1 and R2, respectively. The other end of R1 is connected to terminal 24 and the other end of R2 is connected to terminal 26. As depicted in FIG. 2, V1 supplies a positive voltage pulse through R1 to cable 28 and V2 supplies a negative pulse through R2 to cable 28.
The nominal value of Z0 is typically 100 ohms. Values of R1 and R2 are typically selected to be 50 ohms. In this way the impedance of the TDR circuit 22 is matched to the characteristic impedance Z0 of link cable 28.
Also connected to terminals 24, 26, is amplifier 30, with its positive input connected to terminal 24 and negative input connected to terminal 26. The output signal of amplifier 30 is illustrated by waveform 70 depicted in FIG. 3.
During TDR testing of cable 28, voltages V1, V2 supply a pulse signal to link cable 28. A fault anywhere along cable 28 results in a reflected waveform that will be detected by TDR circuit 22. The reflected signal is applied to amplifier 30 whose output generates waveform 70 in FIG. 2.
In many circumstances, it is desirable to be able to test link cables without disconnecting them from the network. This saves technicians time and also reduces the chance of error when reconnecting the link cable. Unfortunately, prior art cable testers utilizing TDR circuits of the type depicted in FIG. 2 and described above, do not provide clear and simple testing solutions for cables that are connected to a network. This shortcoming of prior art TDR test methods is due to the matching of impedances of the instrument and the cable under test. While this fixed impedance matching is preferable in most testing methodologies, it is actually detrimental when testing terminated network cables. It is well known that matching these impedances results in minimal power loss and, therefore, produces the strongest signals during test by minimizing return signals. However, when the cable under test is terminated into a network, the mismatched impedance of the connection results in a small or non-existent return signal, which in turn reduces the effectiveness of the TDR test.
This problem is further illustrated by waveform 70 depicted in FIG. 3, which represents the reflected signal for a link cable that is tested while connected into a network. The portion A of the waveform illustrates the reflected signal identifying of the end of the link cable. As is clear from this portion of FIG. 3, the TDR reflection indicating the end of the link cable, and hence the length of the cable, is difficult to detect. It would be very difficult for a cable test technician to extract information from the reflected signal of FIG. 3 if it were to appear on a display in a hand held instrument of the sort typically used in the field.
Accordingly, there is a need to provide an apparatus and method for TDR testing of a link cable while it is connected to the network. The apparatus and method should be simple and easily embodied into handheld test instruments so that cable test technicians may readily interpret the results provided by the instrument. The present invention is directed to an apparatus and method designed to achieve these results.